This invention relates to differential amplifiers in general, and in particular to a differential amplifier with dynamic thermal balancing.
Thermal distortion of signals through differential amplifiers results from temperature changes caused by the signal modulating the instantaneous power of the active devices, e.g., transistors, of the differential amplifier. These temperature changes cause the device parameters to change, in turn distorting the signals. Thermal distortion in solid-state devices is a problem well known to circuit designers, and attempts to solve this problem range from simply reducing signals (to reduce power in the devices) to various compensation and error-correction circuits added to the circuit.
A particularly popular differential amplifier for use in electronic instruments is the integrated circuit gain cell taught by Gilbert in U.S. Pat. No. 3,689,752, which is assigned to the assignee of the present invention. The utility of the Gilbert gain cell as a variable gain amplifier is well known. Here, a differential input signal is applied to two pairs of emitter-coupled transistors, while a gain control signal is applied to a pair of current-sourcing transistors to control the current through the amplifying transistors and thus modulate the amplified signal.
One solution to the foregoing problem is taught by Arthur J. Metz in U.S. Pat. No. 4,340,866, which is assigned to the assignee of the present invention. Metz added a pair of compensation transistors connected to oppose the outputs of a pair of gain-controlled transistors. While this method of correcting thermal distortion is effective, it would be desirable to maintain thermal balance between the two sides of the differential amplifier rather than to correct for changes in thermal balance.
In looking at the problem of thermal balance, it is to be noted that the collectors of the amplifying transistors are connected to a source of voltage V.sub.CC through thermal balancing resistors R.sub.L, and thermal balance is attained when (I.sub.TOTAL)*(1/2R.sub.L)=(V.sub.CC -V.sub.E), wherein I.sub.TOTAL is the total current provided to the amplifying transistors. In this condition, maximum power is dissipated in the transistors when I.sub.1 =I.sub.2, that is, when the current through the two sides of the differential amplifier is equal, and power dissipation decreases in both sides of the amplifier equally with instantaneous input signal swing. If I.sub.TOTAL is changed, for example, by selecting a new gain setting, the thermal balance is upset.